An anonymous reader writes: In 1955, John Nash sends an amazing letter to the NSA in order to support an encryption design that he suggested. In it he no less than anticipates computational complexity theory as well as modern cryptography.

In the letter he proposes that the security of encryption can be based on computational hardness and makes the distinction between polynomial time and exponential time: "So a logical way to classify enciphering processes is by the way in which the computation length for the computation of the key increases with increasing length of the key. This is at best exponential and at worst probably at most a relatively small power of r, ar^2 or ar^3, as in substitution ciphers.Link to Original Source